1. Technical Field
The present invention relates to an ink set with an extended color reproduction range (gamut), and particularly to an ink set having an extended color reproduction range in a blue region of a dark portion and excellent color reproducibility in other color regions. Also, the present invention relates to a recording apparatus and recording method using the ink set.
2. Related Art
A CIE/L*a*b* color system is known as a color model. According to the CIE/L*a*b*, all colors (spectra) in a visible light region which can be observed by the human eye can be indicated by chromaticity coordinates.
Among the three parameters (L*, a*, b*) of the CIE/L*a*b* color system, L* represents luminance (brightness) and a* and b* represent chromaticity showing hue and saturation. When L*=0, a black color is indicated, and when L*=100, a white color is indicated. In addition, a* represents a position between red and green colors, and negative value and positive value indicate green and red, respectively. Further, b* represents a position between yellow and blue colors, and negative value and positive value indicate blue and yellow, respectively. Since the L*a*b* color model is tree-dimensional, a color space is expressed by a three-dimensional space in which luminance is represented on the vertical axis.
However, the L*a*b* color model is merely an index used for conceptually explaining a color space, and the color reproduction range is limited. Namely, for example, when an image is formed on a recording medium using a pigment-based ink or ink set, a color which cannot be reproduced by a color of the L*a*b* color model is present.
On the other hand, ink compositions containing various pigments with high color development have been proposed. Japanese Unexamined Patent Application Publication No. 2006-282810 discloses that an ink having high color development and excellent ejection stability, storage stability, and ink reliability for clogging and the like is prepared using an ink composition containing a quinacridone solid solution pigment, a polyether-modified polydimethylsiloxane compound, and water. In addition, Japanese Unexamined Patent Application Publication No. 2007-186641 discloses that an ink having excellent printability, ejection stability, storage stability, and ink reliability for clogging and the like is realized using an ink composition containing a surface-treated pigment (yellow, magenta, cyan) including hydrophilic dispersive groups bonded to the surfaces of a solid-solution pigment composed of two or more specified pigments, a water-insoluble vinyl polymer, and water.
However, in particular, in a blue region of a dark portion having a L* value of 20 or less, color reproduction is limited. That is, when luminance is low, chromaticity and saturation are not easily expressed, and thus, for example, a color with an L* value of 20 or less, an a* value of 30 or more, and a b* value of −75 or less, such as (L*, a*, b*)=(19.4, 30.0, −72.6) or (L*, a*, b*)=(19.4, 37.6, −76.8) in the CIE/L*a*b* color system, cannot be reproduced on an image recording medium using droplets (ink). Therefore, the inventors intensively studied in order to extend a color reproduction range in a blue region of a dark portion. As a result, the inventors have proposed in Japanese Patent Application No. 2008-215056 as prior application that the color reproduction range in a blue region of a dark portion with a L* value of 20 or less can be extended by an ink set including a combination of a cyan ink composition containing C. I. Pigment Blue 15:6 or C. I. Pigment Blue 60 as a pigment and a magenta ink composition containing a quinacridone pigment. In particular, it has been proposed that a color with a L* value of 20 or less, an a* value of 30 or more, and a b value of −75 or less, which has so far been impossible to reproduce, can be realized by an ink set including a combination of a cyan ink composition containing C. I. Pigment Blue 15:6 as a pigment and a magenta ink composition containing a quinacridone solid solution pigment.
On the other hand, with respect to an ink set, generally, a blue region color is reproduced by mixing a cyan ink composition and a magenta ink composition, a red region color is reproduced by mixing a yellow ink composition and a magenta ink composition, and a green region color is reproduced by mixing a yellow ink composition and a cyan ink composition. Therefore, when the color reproduction range in a blue region is extended, it is necessary to take account of color reproducibility in a green region which is reproduced by a cyan ink composition and a yellow ink composition and in a red region which is reproduced by a yellow ink composition and a magenta ink composition. Namely, it is necessary to simultaneously satisfy the color reproduction ranges in the blue region and the other color regions. On the other hand, the inventors have found that although C. I. Pigment Blue 60 contained in a cyan ink composition is an effective pigment type for enlarging the reproduction range in the blue region of a dark portion, the pigment significantly decreases the color development in the green region. Therefore, it has been proposed in Japanese Patent Application No. 2008-232165 that reproducibility in both the blue region and the green region can be realized by an ink set including a combination of a cyan ink composition containing C. I. Pigment Blue 15:6 as a pigment, a magenta ink composition containing a quinacridone pigment selected from the group consisting of C. I. Pigment Violet 19, C. I. Pigment Red 122, C. I. Pigment Red 202, and C. I. Pigment Red 209 or a quinacridone solid solution pigment composed of at least two selected from the group, and a yellow ink composition containing a pigment selected from the group consisting of C. I. Pigment Yellow 74, C. I. Pigment Yellow 213, and C. I. Pigment Yellow 185.
However, the inventors have recently found that C. I. Pigment Blue 15:6 contained as a pigment in a cyan ink composition has poor graininess for recoding an image in a high-brightness region. An ink set is generally composed of a cyan ink composition, a magenta ink composition, a yellow ink composition, a light cyan ink composition, and a light magenta ink composition. In order to improve gradation and graininess, an ink composition (light ink composition) at a lower pigment concentration among the similar color ink compositions in the ink set is used for recording an image in a high-brightness region. However, it has been found that a problem with graininess, such as noticeable dots or the like, occurs in a high-brightness cyan region and green region in which an image is recorded using a light cyan composition containing C. I. Pigment Blue 15:6 as a pigment.